If you have all the proper documents, no need to worry then, sometimes this stuff just happens. You've heard some anecdotes from others above, but I'll throw in one more for good measure: the other day, I had this same problem when flying YYC-IAH-BOG (AC-UA) on a UA ticket booked by our corporate TA. I had not had that problem on any of my half-dozen identical trips in the past year, including one just a few weeks ago. Sometimes it just seems to be random as I was not SSSS or anything else funky.

Just show up at the airport prior to check-in closing time and you'll be fine.